1. Field of the Invention
This invention relates to high-frequency electronic circuit technology, and more particularly, to a ground-plane slotted type signal transmission circuit board which is designed for use with a high-speed digital signal processing system for providing a low-loss signal transmission function.
2. Description of Related Art
With the advent of wireless digital communication technologies, such as wireless networking, mobile phones, GPS (Global Positioning System), etc., the design and manufacture of high-speed digital circuit boards that handle digital signals within the gigahertz range is in high demand in the electronics industry. In circuit layout design, high-speed digital circuit boards typically use microstrips or striplines as signal lines for conveying digital signals within the gigahertz frequency range.
Presently, the electronic industry has developed various different types of high-speed digital signal transmission circuit board structures. FIG. 1 and FIG. 2 show two examples of conventional circuit board structures that are widely utilized for transmission of high-speed digital signals. For description of theory and principle of these two conventional types of high-speed digital signal transmission circuit board structure, please referred to the technical paper entitled “Novel CMOS Low-loss Transmission Line Structure” authored by Jaewon Kim et al and published on IEEE Radio and Wireless Conference, pp. 235-238, September 2004.
FIG. 1 shows the structure of a conventional GCPW (Grounded Coplanar Waveguide) type of circuit board structure 100. As shown, this GCPW circuit board structure 100 comprises: (A) a substrate 110; (B) a signal line 120; and (C) a pair of grounding lines 130. In this GCPW circuit board structure 100, the substrate 110 is made of a dielectric material with a front surface 111 and an opposite back surface 112; wherein the opposite back surface 112 is coated with an electrically conductive material to serve as a ground plane 113. The signal line 120 is laid over the front surface 111 of the substrate 110 for use as a signal conveying line. The grounding lines 130 are printed circuit lines laid over the front surface 111 of the substrate 110 extending alongside the signal line 120. In practice, the signal line 120 can be used for transmission of high-speed digital signals.
In practical applications, however, since the thickness of the dielectric material of the substrate 110 (i.e., the distance between the front surface 111 and the back surface 112) is very small, it could easily allow the electric fields of the high-speed digital signals being transmitting over the signal line 120 to cause an induced current, thus undesirably resulting in a leakage current that flows from the signal line 120 through the substrate 110 to the ground plane 113 on the back surface 112. This leakage current will then cause a large attenuation to the high-speed digital signals.
FIG. 2 shows the structure of a conventional semi-coaxial type of circuit board structure 200. As shown, this semi-coaxial type circuit board structure 200 comprises: (A) a substrate 210; (B) a signal line 220; (C) a pair of grounding lines 230; and (D) a stack-type via structure 240. The semi-coaxial type circuit board structure 200 shown in FIG. 2 differs from the GCPW circuit board structure 100 shown in FIG. 1 only in the addition of the stack-type via structure 240, which is composed of an electrically conductive pillar 241 and a plurality of stacked electrically-conductive planes 242; wherein the electrically conductive pillar 241 extends from the front surface 211 to the back surface 212 of the substrate 210; while the stacked electrically-conductive planes 242 are substantially parallel to the front surface 211 and the back surface 212 of the substrate 210, and the multiple planes thereof are gradually increased in widths from top to bottom to form a tapered ladder-like structure which is functionally equivalent to a semi-coaxial transmission line.
However, the semi-coaxial type circuit board structure 200 shown in FIG. 2 is fundamentally similar to the GCPW circuit board structure 100 shown in FIG. 1. As a consequence, the issue of leakage current induced in the ground plane still exists.